Turbine engines, and particularly gas or combustion turbine engines, are rotary engines that extract energy from a flow of combusted gases passing through the engine onto a multitude of turbine blades. Gas turbine engines comprise a core formed of a compressor section, combustion section, and turbine section. The compressor section and turbine section comprise compressor blades and turbine blades that are mounted to a common drive shaft, and are collectively referred to as a rotor, which is surrounded by a casing. In some gas turbine engines there are multiple rotors, such as a low pressure rotor and a high pressure rotor, with the drive shafts being coaxial.
Gas turbine engines have been used for land and nautical locomotion and power generation, but are most commonly used for aeronautical applications such as for aircraft, including helicopters. In aircraft, gas turbine engines are primarily used for propulsion of the aircraft, along with power generation. In terrestrial applications, turbine engines are primarily used for power generation.
Gas turbine engines for aircraft are designed to operate at high temperatures, approximately 2000° C., to maximize engine efficiency, therefore cooling of certain engine components may be necessary. Typically, cooling is accomplished by ducting cooler air, approximately 900° C., from the high and/or low pressure compressors to the engine components which require cooling.
When the turbine engine is turned off after operating at such high temperatures, the heat stratifies in the engine core and the top of the rotor will become hotter than the bottom due to rising heat. The stratification can often lead to a 500° C. difference between the top and bottom of the rotor, which leads to asymmetrical thermal expansion between the top and bottom of the rotor. Under such a temperature difference, the top of the rotor thermally expands a greater radial amount causing what is referred to as a bowed-rotor condition. The bowed-rotor condition may occur within 10 minutes of the engine being turned off and may last up to 8 hours.
The asymmetrical thermal expansion moves the center of mass upward and out of alignment with the rotational axis of the shaft, resulting in an out of balance condition when the rotor is turned. This upward movement also reduces the clearance between the tips of the blades and the casing. When the rotor is again rotated, the out of balance caused will cause vibration during rotation. The vibrations will increase rotor deflections, especially when passing through a vibratory mode, such as a rotational natural frequency for the rotor. The vibrations can accelerate normal cracking and fatigue, leading to earlier and more frequent maintenance. They also can accelerate wear and tear on seals and similar structures.
The thermal expansion may be great enough that when the upper part of the rotor is rotated to the lower part of the surrounding engine casing, it may contact the lower portion of the casing, which has not radially expanded as much as the upper portion of the engine casing. The repeated contact or rubbing with the engine casing during rotation of the rotor with a bowed-rotor condition may cause parts to break off and thus may cause foreign object damage to the engine.
Prior solutions to the bowed rotor phenomena are directed to prevention and mitigation once the bow has occurred. The most common current solution is to rotate the engine with the starter for an extended period when it is desired to start the engine until the bow dissipates. This can take several minutes and has several undesirable effects. First, there may be uncomfortably loud cabin noise due to the rotor vibration being carried through the aircraft structure. Second, the aircraft auxiliary power unit life is consumed due to the longer time at high power required to rotate the engine starter. Third, the added starting time causes airport congestion as the aircraft must remain in the taxiway during engine start, blocking other aircraft movements. Fourth, if the engine rotor has a natural frequency in the speed range of the starting sequence, the damage described above can occur. Previous solutions to these problems have proposed to externally rotate the rotor at a low speed, approximately 1 rpm until the bowed rotor phenomena disappears, which is often more than a half an hour, which is an unacceptable downtime in the operation of aircraft, especially commercial airliner.
Typically, upon shutting off the turbine engine, the rotor would be rotated at low speed, approximately 1 rpm, by an external power source, such as a pneumatic drive or electric motor, to prevent the bowed rotor. Such preventative action requires an additional step to the shutdown of the engine, which may be forgotten by a ground crew. Alternatively, the turbine engine would not be shut down, which consumed relatively substantial amounts of fuel. The bowed rotor phenomena can occur as quickly as within 10 minutes of shut down. Thus, even if a ground crew takes action to slowly rotate the engine, they may not act fast enough.
Once the bowed-rotor condition is present, it may naturally last for up to 8 hours, which is an undesirably long time for the aircraft to be out of operation. Thus, given the relatively short time needed for the bowed-rotor condition to arise and the relatively long time for it to naturally subside, it is important to prevent or address the bowed rotor phenomena for normal operation of the aircraft. Otherwise, once the bowed rotor phenomena occurs, the most common current solution is to rotate the engine with the starter for an extended period until the bow dissipates. This can take several minutes and has several undesirable effects. First, there may be uncomfortably loud cabin noise due to the rotor vibration being carried through the aircraft structure. Second, the aircraft auxiliary power unit life is consumed due to the longer time at high power required to rotate the engine starter. Third, the added starting time causes airport congestion as the aircraft must remain in the taxiway during engine start, blocking other aircraft movements. Fourth, if the engine rotor has a natural frequency in the speed range of the starting sequence, the damage described above can occur. Previous solutions to these problems have proposed to externally rotate the rotor at a low speed, approximately 1 rpm, until the bowed rotor phenomena disappears, which is often more than a half an hour, which is an unacceptable downtime in the operation of aircraft, especially commercial airliner.